1. Field of the Invention
This invention relates to a gas concentration sensing apparatus using a gas concentration sensor which detects a concentration of a specific component of a measurement gas such as an exhaust gas emitted from an automotive engine.
2. Description of the Related Art
In general, a NOx concentration sensor is necessary to implement feedback control of an automotive engine to reduce NOx emission therefrom. A NOx concentration sensor located at a point in an engine exhaust passage downstream of a NOx-processing catalytic converter can be used in a system for determining whether or not the catalytic converter has significantly deteriorated.
U.S. Pat. No. 5,866,799 corresponding to Japanese published unexamined patent application 8-271476 discloses a device for measuring a concentration of a gas component (NOx) of a measurement gas. In the device of U.S. Pat. No. 5,866,799, the measurement gas containing the gas component is introduced from an external measurement-gas space into a first internal space under a diffusion resistance. An amount of oxygen in the measurement gas within the first internal space is controlled so as to produce an atmosphere which does not substantially affect measurement of the gas component and which does not convert the gas component. The atmosphere is introduced from the first internal space into a second internal space under a diffusion resistance. Measurement is made as to the concentration of the gas component present in the atmosphere in the second internal space.
European patent application EP 0798555 A2 corresponding to Japanese published unexamined patent application 9-318596 discloses an oxide sensor (a NOx sensor) in which a measurement gas is introduced into a first chamber through a first diffusion rate-determining section, and O2 is removed from the measurement gas by the aid of a pumping voltage applied between pumping electrodes. Then, the measurement gas is introduced into a second chamber through a second diffusion rate-determining section to remove excessive O2 by the aid of auxiliary pumping electrodes. A detecting electrode is arranged to satisfy xe2x80x9cdxe2x89xa7txe2x80x9d provided that xe2x80x9cdxe2x80x9d represents a distance from an end of the auxiliary pumping electrodes on the side of the second diffusion rate-determining section to an end of the detecting electrode on the side of the second diffusion rate-determining section, and xe2x80x9ctxe2x80x9d represents a height of the second chamber. A predetermined pumping voltage is applied to the detecting electrode to decompose oxides contained in the measurement gas by the aid of the detecting electrode or a catalyst. The amount of oxygen produced by the decomposition is measured to determine the concentration of the oxides.
It is desirable that NOx concentration sensing apparatuses used in exhaust systems of engines to detect the NOx concentrations in exhaust gases and also the air-to-fuel ratios of air-fuel mixtures have wide sensible ranges of the air-to-fuel ratios. Also, it is desirable that the NOx concentration sensing apparatuses have good response characteristics.
It is an object of this invention to provide a gas concentration sensing apparatus which has a wide sensible range and a good response characteristic.
A first aspect of this invention provides a gas concentration sensing apparatus comprising a gas introducing portion for introducing a measurement gas; a first cell opposed to the gas introduction portion for pumping oxygen from the measurement gas in the gas introducing portion; a second cell opposed to the gas introducing portion for sensing a concentration in a specific component of the measurement gas from which oxygen has been pumped by the first cell; a reference gas chamber to which at least one of the first cell and the second cell is exposed, wherein the one of the first cell and the second cell which is exposed to the reference gas chamber includes an electrode facing the gas introducing portion; means for sensing a current flowing through the first cell; means for applying a voltage to the first cell in response to the sensed current through the first cell; means for sensing a current flowing through the second cell; means for applying a voltage to the second cell in response to the sensed current through the second cell; and means for floating a voltage at the electrode from a voltage of 0 V.
A second aspect of this invention provides a gas concentration sensing apparatus comprising a gas introducing portion for introducing a measurement gas; a first cell opposed to the gas introduction portion for pumping oxygen from the measurement gas in the gas introducing portion, the first cell including a positive-side electrode and a negative-side electrode; a second cell opposed to the gas introducing portion for sensing a concentration in a specific component of the measurement gas from which oxygen has been pumped by the first cell, the second cell including a positive-side electrode and a negative-side electrode; means for sensing a current flowing through the first cell; means for applying a voltage to the first cell in response to the sensed current through the first cell; means for sensing a current flowing through the second cell; means for applying a voltage to the second cell in response to the sensed current through the second cell; and means for floating a voltage at the negative-side electrode of the first cell and a voltage at the negative-side electrode of the second cell from a voltage of 0 V.
A third aspect of this invention is based on the first aspect thereof, and provides a gas concentration sensing apparatus wherein the floating means comprises means for applying a predetermined reference voltage to the electrode, the predetermined reference voltage being higher than the voltages which are applied to the first cell and the second cell when the measurement gas lacks oxygen.
A fourth aspect of this invention is based on the first aspect thereof, and provides a gas concentration sensing apparatus further comprising a single power supply, and means for deriving the voltages applied to the first cell and the second cell from the single power supply.
A fifth aspect of this invention is based on the first aspect thereof, and provides a gas concentration sensing apparatus further comprising an automotive battery, and means for deriving the voltages applied to the first cell and the second cell from the automotive battery, the measurement gas including an automotive exhaust gas.
A sixth aspect of this invention is based on the first aspect thereof, and provides a gas concentration sensing apparatus further comprising means for detecting an impedance of the first cell, and means for detecting an impedance of the second cell.
A seventh aspect of this invention is based on the first aspect thereof, and provides a gas concentration sensing apparatus wherein the means for sensing the current through the first cell includes a first current sensing resistor, and means for detecting a voltage across the first current sensing resistor, and wherein the means for sensing the current through the second cell includes a second current sensing resistor, and means for detecting a voltage across the second current sensing resistor.
An eighth aspect of this invention is based on the first aspect thereof, and provides a gas concentration sensing apparatus wherein the measurement gas includes an exhaust gas, and further comprising means for controlling the voltage applied to the other of the first cell and the second cell which is not exposed to the reference gas chamber to suspend transfer of oxygen via the other of the first cell and the second cell when the exhaust gas is caused by a rich air-fuel mixture.
A ninth aspect of this invention is based on the first aspect thereof, and provides a gas concentration sensing apparatus wherein the measurement gas includes an exhaust gas, and further comprising a switch circuit interposed in a voltage feed path to the other of the first cell and the second cell which is not exposed to the reference gas chamber, and means for opening the switch circuit to suspend transfer of oxygen via the other of the first cell and the second cell when the exhaust gas is caused by a rich air-fuel mixture.
A tenth aspect of this invention provides a gas concentration sensing apparatus comprising a first cell for pumping O2 from a first exhaust gas to change the first exhaust gas to a second exhaust gas; a second cell for decomposing NOx in the second exhaust gas and thereby generating new O2, and for pumping the new O2 from the second exhaust gas to a reference gas; means for applying a voltage across the second cell; and means for changing a polarity of the voltage between a positive state and a negative state.
An eleventh aspect of this invention provides a gas concentration sensing apparatus comprising a first cell for pumping O2 from a first exhaust gas to a reference gas to change the first exhaust gas to a second exhaust gas; a second cell for decomposing NOx in the second exhaust gas and thereby generating new O2, and for pumping the new O2 from the second exhaust gas; means for applying a voltage across the first cell; and means for changing a polarity of the voltage between a positive state and a negative state.